Flow measurement and control

ABSTRACT

An exhaust gas recirculation systems directs exhaust gasses from an exhaust manifold to an intake manifold of an internal combustion engine. The exhaust gasses travel from the exhaust manifold, first passing through a flow control valve and then through a measuring orifice before entering the intake manifold. Pressure difference across the orifice is used, along with correction factors based on the pressure difference and pressure downstream of the orifice, to measure and control exhaust gas flow.

FIELD OF THE INVENTION

The present invention relates to a system and method to measure andcontrol gas flow using pressure measurements upstream and downstream ofan orifice, and in particular to measurement of exhaust gasrecirculation flow with a flow control valve upstream of the orifice.

BACKGROUND OF THE INVENTION

Engine control systems require accurate control of exhaust gasrecirculation (EGR) for controlling regulated emissions and achievingfuel economy improvements. One type of exhaust gas recirculation systemexternally recirculates the exhaust gas from the exhaust manifold to theintake manifold with a flow control valve placed in the flow pathbetween the exhaust manifold and the intake manifold. Typically, thevalve is pneumatically operated and controlled by an electronic enginecontroller.

One approach to controlling exhaust gas recirculation flow is to use afeedback variable to assure that the actual exhaust gas recirculationflow converges to the desired exhaust gas recirculation flow. One methodis to use a differential pressure measured across an orifice in theexhaust flow path upstream of the flow control valve. Then, thedifferential pressure can be used to infer the actual exhaust gasrecirculation flow. The differential pressure measurement providesadequate correlation to exhaust flow because the exhaust pressure variesonly slightly in the region where EGR is utilized. Further temperatureeffects can be accounted for because the upstream exhaust manifoldtemperature can be correlated to engine operating conditions or ignoreddue to relatively small variations. Finally, an error between the actualand desired exhaust gas recirculation flow is used to create a controlsignal that is sent to the flow control valve. Thus, the system cancompensate for the effects of engine and component aging, as well asother errors in the system. Such a system is disclosed in U.S. Pat. No.5,190,017.

The inventors herein have recognized a disadvantage with the abovesystem when the orifice is placed downstream of the valve. In thisconfiguration, flow from the exhaust travels first through a flowcontrol valve and then through the orifice before entering the intakemanifold. In this case, the pressure upstream of the orifice (downstreamof the valve) varies widely and the assumptions made regardingdifferential pressure and flow are no longer valid. Also, thetemperature upstream of the orifice (downstream of the valve) is nolonger correlated directly to engine operating conditions due to theflow expansion in the valve. Thus, there is a significant measurementerror when using a differential pressure measurement with a downstreamorifice.

One approach to more accurately measure flow is to measure absolutepressure upstream of the orifice, pressure differential across theorifice, and temperature upstream of the orifice. In this way, acorrelation between the pressures and temperature can be used to measurethe exhaust flow where the pressure and temperatures are widely varying.Alternatively, this approach can be used with the flow control valvewhere pressure upstream of the valve, pressure differential across thevalve, temperature upstream of the valve, and valve area are used tomeasure flow. Such a system is disclosed in U.S. Pat. No. 4,406,161.

The inventors have recognized a disadvantage with the above approach.The approach requires that upstream temperature be known. Thus, a sensoris needed which adds additional cost and is unacceptable. Further,exhaust manifold temperature estimates based on engine operatingconditions inaccurately represent the temperature downstream of a flowcontrol valve. Further, with application of prior art approaches to theflow control valve, valve position, or area, must be measured, addingadditional cost.

SUMMARY OF THE INVENTION

An object of the invention claimed herein is to provide an exhaust gasrecirculation measurement system and method for an exhaust gasrecirculation system having an upstream flow control valve and adownstream measuring orifice.

The above object is achieved, and problems of prior approaches overcome,by a flow measurement system for measuring exhaust gas flow from anexhaust manifold of an internal combustion engine to an intake manifoldof the engine. The system comprises a flow control valve having avariable orifice positioned in an exhaust gas recirculation path betweenthe exhaust manifold and intake manifold of the engine, a fixed orificearea located in said path and downstream of said valve, and a computerfor measuring a first pressure downstream of said measuring orifice,measuring a differential pressure across said measuring orifice, andcalculating a mass flow based on said first pressure and saiddifferential pressure.

By using a combination of the differential pressure across the orificeand a correction as a function of an absolute pressure downstream of theorifice, wherein the orifice is downstream of a flow control valve, ameasurement of flow is obtained that gives acceptable performance andavoids discrepancies of prior approaches. Stated another way, anapproximation using pressure differential across the orifice andabsolute pressure downstream of the orifice accurately measures flow.Temperature measurement upstream of the orifice (downstream of the flowcontrol valve) is inherently included. This measurement method isjustified for the special case of flow through an orifice locateddownstream of a valve, wherein flow originates from an exhaust manifoldof an internal combustion engine an exits to an intake manifold of theengine.

This embodiment, herein referred to as the first embodiment, uses theproduct of pressure differential across the orifice and a correctionfactor, where the correction factor is related to the pressuredownstream of the orifice.

An advantage of the above aspect of the invention is that more accuratefeedback control of EGR is obtained.

Another advantage of the above aspect of the invention is that the moreaccurate feedback control quality yields better fuel economy anddriveability.

Yet another advantage of the above aspect of the invention is that theconsistent feedback control quality yields lower emissions.

Still another advantage of the above aspect of the invention is that thepressure measurement downstream of the measuring orifice serves the dualpurpose of forming a correction factor for the EGR flow and measuringmanifold pressure for other uses.

In another aspect of the invention, a second correction is used tofurther improve the measurement system by a method for measuring flowfrom an engine exhaust to and engine intake wherein the flow passesthrough a flow control valve and then a fixed area measuring orifice.The method comprises measuring a pressure difference across themeasuring orifice, measuring a pressure downstream of the measuringorifice representative of manifold pressure, calculating a pressure andtemperature correction based on said downstream pressure and saiddifferential pressure, and calculating a flow based on said downstreampressure, said differential pressure, and said correction.

In the configuration where the flow control valve is placed upstream andthe measuring orifice is placed downstream, both between in the exhaustmanifold and intake manifold, a further approximation using manifoldpressure and pressure drop across the measuring orifice can be found toinclude both pressure and temperature effects due to compressible flowexpansion through the valve. This embodiment, herein referred to as thesecond embodiment, obtains further improved accuracy over the firstembodiment previously described herein with no additional sensors. Inthe second embodiment, there is an additional correction factor usedthat is a function of both pressure difference across the orifice andpressure downstream of the orifice. Again, temperature measurementupstream of the orifice (downstream of the flow control valve) isunnecessary.

An advantage of the above aspect of the invention is that more accuratefeedback control of EGR is obtained.

Another advantage of the above aspect of the invention is that the moreaccurate feedback control quality yields better fuel economy anddriveability.

Yet another advantage of the above aspect of the invention is that theconsistent feedback control quality yields lower emissions.

In another aspect of the invention, the above object is achieved, andproblems of prior approaches overcome, by an article of manufacturecomprising a housing, a flow control valve contained in said housing,said flow control valve having a variable area orifice disposed within agas flow passage and connected to an inlet portion of said passage, afixed area orifice disposed within said passage and connected to anoutlet portion of said passage, a first differential pressure sensorcoupled across said fixed area orifice to measure a differentialpressure across said fixed area orifice, and a second pressure sensorcoupled to said outlet portion to measure outlet pressure.

An advantage of the above aspect of the invention is that more accuratefeedback control of flow is obtained.

Other objects, features and advantages of the present invention will bereadily appreciated by the reader of this specification.

BRIEF DESCRIPTION OF THE DRAWINGS

The object and advantages described herein will be more fully understoodby reading an example of an embodiment in which the invention is used toadvantage, referred to herein as the Description of the PreferredEmbodiment, with reference to the drawings wherein:

FIG. 1 is a block diagram of an engine in which the invention is used toadvantage;

FIGS. 2A and 2B are alternate embodiments of the present invention; and

FIGS. 3-5 are a high level flowcharts of various routines forcontrolling EGR flow.

DESCRIPTION OF AN EMBODIMENT

Internal combustion engine 10 comprising a plurality of cylinders, onecylinder of which is shown in FIG. 1, is controlled by electronic enginecontroller 12. Engine 10 includes combustion chamber 30 and cylinderwalls 32 with piston 36 positioned therein and connected to crankshaft40. Combustion chamber 30 communicates with intake manifold 44 andexhaust manifold 48 via respective intake valve 52 and exhaust valve 54.Exhaust gas oxygen sensor 16 is coupled to exhaust manifold 48 of engine10 upstream of catalytic converter 20.

Intake manifold 44 communicates with throttle body 64 via throttle plate66. Intake manifold 44 is also shown having fuel injector 68 coupledthereto for delivering fuel in proportion to the pulse width of signal(fpw) from controller 12. Fuel is delivered to fuel injector 68 by aconventional fuel system (not shown) including a fuel tank, fuel pump,and fuel rail (not shown). Engine 10 further includes conventionaldistributorless ignition system 88 to provide ignition spark tocombustion chamber 30 via spark plug 92 in response to controller 12. Inthe embodiment described herein, controller 12 is a conventionalmicrocomputer including: microprocessor unit 102, input/output ports104, electronic memory chip 106, which is an electronically programmablememory in this particular example, random access memory 108, and aconventional data bus.

Controller 12 receives various signals from sensors coupled to engine10, in addition to those signals previously discussed, including:measurements of inducted mass air flow (MAF) from mass air flow sensor110 coupled to throttle body 64; engine coolant temperature (ECT) fromtemperature sensor 112 coupled to cooling jacket 114; a measurement ofmanifold pressure (MAP) from manifold pressure sensor 116 coupled tointake manifold 44; a measurement of throttle position (TP) fromthrottle position sensor 117 coupled to throttle plate 66; and a profileignition pickup signal (PIP) from Hall effect sensor 118 coupled tocrankshaft 40 indicating and engine speed (N).

Exhaust gas is delivered to intake manifold 44 by a conventional EGRtube 202 communicating with exhaust manifold 48, EGR valve assembly 200,and EGR orifice 205. Alternatively, tube 202 could be a internallyrouted passage in the engine that communicates between exhaust manifold48 and intake manifold 44. Flow Sensor 206 communicates with EGR tube202 between valve assembly 200 and orifice 205. Flow sensor 206 alsocommunicates with intake manifold 44. Stated another way, exhaust gastravels from exhaust manifold 44 first through valve assembly 200, thenthrough EGR orifice 205, to intake manifold 44. EGR valve assembly 200can then be said to be located upstream of orifice 205.

Flow sensor 206 provides a measurement of manifold pressure (MAP) andpressure drop across orifice 205 (DP) to controller 12. Signals MAP andDP are then used to calculated EGR flow as described later herein withparticular reference to FIGS. 3-5. EGR valve assembly 200 has a valveposition (not shown) for controlling a variable area restriction in EGRtube 202, which thereby controls EGR flow. EGR valve assembly 200 caneither minimally restrict EGR flow through tube 202 or completelyrestrict EGR flow through tube 202. Vacuum regulator 224 is coupled toEGR valve assembly 200. Vacuum regulator 224 receives actuation signal(226) from controller 12 for controlling valve position of EGR valveassembly 200. In a preferred embodiment, EGR valve assembly 200 is avacuum actuated valve. However, as is obvious to those skilled in theart, any type of flow control valve may be used, such as, for example,an electrical solenoid powered valve or a stepper motor powered valve.

Referring now to FIGS. 2A and 2B, and in particular to FIG. 2A, analternative embodiment of the present invention is shown in whichhousing 250 contains path 252 with inlet end 254 and outlet end 256.Variable orifice 258 is controlled by pintle 260 of valve 200. Housing250 also holds vacuum regulator 224 which is coupled to valve 200 andthereby regulates pintle 260. Path 252 also has orifice 205 coupled tooutlet end 256. Differential pressure sensor 262 measures pressuredifference across orifice 205 and provides differential pressure signal266 to circuit 268. Pressure sensor 264 measures communicates viameasurement path 269 with outlet end 256 and measure pressure downstreamof orifice 205 and provides pressure signal 270 to circuit 268. Circuit268 calculates, either digitally using microprocessor circuits known tothose skilled in the art or using analog circuits known to those skilledin the art, the product of signals 266 and 270. Circuit 268 then makesthe result of this calculation available in signal 272.

Alternatively, as shown in FIG. 2B, differential sensor 262 and sensor264 communicate with downstream flow (not shown) via secondcommunication path 274. In this embodiment, paths 256 and 274 areadapted to be connected to an intake manifold of an internal combustion.Then, path 274 and 256 will be in fluid communication via the intakemanifold. Such an arrangement is preferable if circuit 268 also providesignal 276 representing the pressure measured by sensor 264.

Referring now to FIG. 3, a routine for calculating EGR flow (EM) isdescribed. In step 210, the signal MAP is read by controller 12 fromsensor 206, giving a measure of pressure downstream of orifice 205.Then, in step 212, the differential pressure, DP, across orifice 205 isread by controller 12 from sensor 206. In step 214, a correction factor,CF1, partially accounting for the compressibility effects of the EGRflow is calculated as the absolute pressure measured by signal MAP.Alternatively, if the downstream pressure measured in step 210 waspressure relative to atmosphere, correction factor CF1 would becalculated as the sum of the pressure relative to atmosphere plus theabsolute pressure due to the atmosphere. Then, in step 216, EGR flow,EM, is calculated as the square root of the product of correction factorCF1, differential pressure DP, and constant K. Constant K represents acalibration term that accounts for various unit conversions and the areaof orifice 205. In this way, pressure and temperature effects due to theexpansion of the EGR flow through valve 200 are sufficiently removed andmeasurement error is reduced.

The routine described in FIG. 3 exploits the nature of the flow due toexpansion first through flow control valve 200 and then through orifice205, where the source of flow is exhaust manifold 48 and the sink isintake manifold 44 of internal combustion engine 10. Due to the typicalranges of exhaust manifold pressure and temperature and intake manifoldpressure (MAP), EGR flow may be approximated using the product ofpressure difference (DP) across orifice 205 and pressure downstream(MAP) of orifice 205 without need for measuring temperature upstream oforifice 205 (downstream of flow control valve 200).

Referring now to FIG. 4, an alternate routine for calculating EGR flow(EM) is described. In step 310, the signal MAP is read by controller 12from sensor 206, giving a measure of pressure downstream of orifice 205.Then, in step 312, the differential pressure, DP, across orifice 205 isread by controller 12 from sensor 206. In step 314, a correction factor,CF1, partially accounting for the compressibility effects of the EGRflow is calculated as the absolute pressure measured by signal MAP.Alternatively, if the downstream pressure measured in step 310 waspressure relative to atmosphere, correction factor CF1 would becalculated as the sum of the pressure relative to atmosphere plus theabsolute pressure due to the atmosphere. Then, in step 316, correctionfactor CF2 is calculated as a function of both differential pressure DPand downstream pressure MAP, where k represents the ratio of specificheats of exhaust gas. Correction factor CF2 further accounts for thecompressibility effects of the EGR flow. Then, in step 318, correctionfactor CF3 is calculated as a function of flow through the engine, MAF.Correction factor CF3 accounts for variations in exhaust pressure.Function h represents a function relating airflow through the engine(MAP) to exhaust pressure and is determined experimentally.Additionally, function h can include a correction for barometricpressure. In other words, the exhaust pressure is calculated as afunction of both MAF and barometric pressure. The effect of barometricpressure on exhaust pressure is also determined experimentally.Barometric pressure can be either measured or estimated using methodsknown to those skilled in the art. Then, in step 320, EGR flow, EM, iscalculated as a function of correction factors CF1, CF2, CF3,differential pressure DP and constant K. In this way, pressure andtemperature effects due to the expansion of the EGR flow through valve200 are further removed and measurement error is further reduced withadditional complexity.

Referring now to FIG. 5, a routine for controlling EGR flow isdescribed. In step 410, the desired EGR flow, DESEM, is calculated as afunction of engine operating conditions that include engine speed(determined from signal PIP) and airflow (MAF). Then, the value of EMcalculated according to either FIG. 3 or 4 is subtracted from DESEM tocreate an error signal, ERROR. Then, in step 414, actuation signal 226is calculated as a function (f) of signal ERROR. In a preferredembodiment, function (f) represents a PID controller. Alternatively,function (f) may represent any type of feedback or feedforwardcontroller known to those skilled in the art.

This concludes the description of the Preferred Embodiment. The readingof it by those skilled in the art would bring to mind many alterationsand modifications without departing from the spirit and scope of theinvention. Accordingly, it is intended that the scope of the inventionbe limited by the following claims.

We claim:
 1. An article of manufacture comprising: a housing; a flowcontrol valve contained in said housing, said flow control valve havinga variable area orifice disposed within a gas flow passage and connectedto an inlet portion of said passage; a fixed area orifice disposedwithin said passage and connected to an outlet portion of said passage;a first differential pressure sensor coupled across said fixed areaorifice to measure a differential pressure across said fixed areaorifice; a second pressure sensor coupled to said outlet portion tomeasure outlet pressure; and a circuit for producing a signalrepresentative of a square root of a product of said differentialpressure and said outlet pressure based on said first sensor and saidsecond sensor.
 2. A method for measuring flow from an engine exhaust toand engine intake wherein the flow passes through a flow control valveand then a fixed area measuring orifice, the method comprising;measuring a pressure difference across the measuring orifice, measuringa pressure downstream of the measuring orifice representative ofmanifold pressure, calculating a pressure and temperature correctionbased on said downstream pressure and said differential pressure; andcalculating a flow based on said downstream pressure, said differentialpressure, and said correction.
 3. The method recited in claim 2 furthercomprising the step of modifying said calculated flow based on engineairflow.
 4. The method recited in claim 2 further comprising the step ofmodifying said calculated flow based on barometric pressure.
 5. Themethod recited in claim 2 further comprising the step of controlling theflow control valve based on said calculated flow.
 6. The method recitedin claim 2 wherein said step of calculating said pressure andtemperature correction further comprises the step of calculating saidpressure and temperature correction based on a function of a sum of saiddownstream pressure and said differential pressure.
 7. The methodrecited in claim 6 wherein said function is a power function, with saidpower related to a property of exhaust gas.
 8. A flow measurement systemfor measuring exhaust gas flow from an exhaust manifold of an internalcombustion engine to an intake manifold of the engines the systemcomprising: a flow control valve having a variable orifice positioned inan exhaust gas recirculation path between the exhaust manifold andintake manifold of the engine; a fixed orifice area located in said pathand downstream of said valve; and a computer for measuring a firstpressure downstream of said fixed orifice area, measuring a differentialpressure across said fixed orifice area, and calculating a mass flowbased on a product of said first pressure and said differentialpressure.
 9. The system recited in claim 8 wherein said computer furthercalculates said flow based on a square root of said product of saidfirst pressure and said differential pressure.
 10. The system recited inclaim 8 wherein said computer further calculates said flow based on theproduct of said first pressure and said differential pressure multipliedby a function of a sum of said first pressure and said differentialpressure.
 11. The system recited in claim 8 wherein said function is apower function, with said power related to a property of exhaust gas.12. The system recited in claim 8 wherein said computer further controlssaid flow control valve based on said calculated mass flow.
 13. Thesystem recited in claim 12 wherein said computer further calculates adesired recirculation flow based on engine operating conditions andcontrols said flow control valve based on said desired recirculationflow and said calculated mass flow.
 14. The system recited in claim 13wherein said computer further calculates said flow based on a product ofsaid first pressure and said differential pressure multiplied by afunction of a sum of said first pressure and said differential pressure.15. The system recited in claim 14 wherein said function is a powerfunction, with said power related to a property of exhaust gas.
 16. Thesystem recited in claim 14 wherein said computer further calculates saidflow based on a square root of said product of said first pressure andsaid differential pressure multiplied by said function of said sum ofsaid first pressure and said differential pressure.